A common reaction in fanfiction about Dragon Ball Super Broly is shock at his raw power. Many fans write about how other characters, like Goku and Vegeta, react when they first see Broly's incredible strength. For example, Goku might be excited to face such a powerful opponent, seeing it as a new challenge in his quest for greater strength.
There is a fanfiction about reactions to the arrival of Broly in the Dragon Ball Super timeline. It showed not just the Saiyans' reactions but also how other races in the universe perceived this extremely powerful Saiyan. It was full of action and emotion as different characters tried to figure out what to do about Broly.
The popularity also comes from the fact that it can expand the universe. Fanfiction writers can create scenarios where characters react to things that might not have been fully explored in the original. They can show how the people on Earth react to the new threats from other universes in Dragon Ball Super, which adds more realism to the story.
Well, it depends on how you define fanfiction. Technically, Dragon Ball Super is an official continuation of the Dragon Ball series. However, it does have elements that feel like fanfiction. For example, some of the power - levels and new forms introduced seem a bit outlandish and more like what a passionate fan might come up with rather than a strictly logical progression from the original story. But it's still an official part of the franchise so it's a bit of a gray area.